In wireless communication, when a signal is transmitted while a transmitting station or a receiving station is moving, deviation (offset) occurs in the frequency of the received signal due to the Doppler shift. In order to demodulate/decode the received signal correctly, it is desirable that the frequency offset is estimated and corrected. In this regard, the higher the movement speed of the transmitting station or the receiving station, the larger the frequency offset, making the technique to estimate the frequency offset important.
FIG. 1 is a diagram illustrating the Doppler shift. Here, it is assumed that a frequency difference fu is set between the uplink frequency and the downlink frequency. That is, when the downlink frequency is fc, the uplink frequency is fc+fu. In addition, it is assumed that a Doppler shift fd occurs on the wireless link.
A wireless base station 101 transmits a downlink signal at the frequency fc. Then, the frequency of the received signal at a user terminal 102 becomes fc+fd due to the influence of the Doppler shift. Here, in cellular communication systems such as the LTE (Long Term Evolution) or the UMTS (Universal Mobile Telecommunications System), the user terminal 102 has an AFC (Automatic Frequency Control) circuit, and adjusts the transmission frequency using the downlink signal received from the wireless base station 101. Therefore, the user terminal 102 transmits an uplink signal at a frequency of fc+fu+fd. Then, the frequency of the received signal at the wireless base station 101 is fc+2fd+fu. That is, a frequency offset “2fd” is generated due to the Doppler shift.
The frequency offset is dependent on the movement speed of the mobile station (in FIG. 1, the user terminal 102) and the frequency band. For example, when the movement speed of the user terminal 102 is 350 km/h and the frequency band of the wireless signal is 2 GHz, the maximum value of the frequency offset “2fd” that occurs in the uplink is about 1300 Hz.
As a method for estimating the frequency offset described above, there is a technique in which a receiver estimates a phase rotation amount based on the correlation between a pilot signal received periodically and a predetermined reference signal, and calculates the frequency offset from the phase rotation amount (for example, WO93/22861).
However, in the conventional arts, the range in which the frequency offset can be estimated is narrow. For example, the message 3 in the LTE system belongs to the PUSCH (Physical Uplink Shared Channel). Here, in the message 3, as illustrated in FIG. 2, a pilot block is inserted at intervals of 0.5 millisecond. The frequency offset can be estimated using the pilot block, however, the frequency offset can be estimated only within the range of about ±1000 Hz. Meanwhile, in a configuration in which frequency hopping is implemented, it is practically impossible to estimate the frequency offset.
As described above, in the conventional arts, the range in which the frequency offset can be estimated has been narrow. Particularly, in a case in which the movement speed of the user terminal is high, it has been difficult to estimate the frequency offset.